1. Field of the Invention
The present invention relates to a device to improve the charging or discharging process of a battery, to increase the usable capacity and the usable lifetime (number of cycles) of a battery.
2. Description of Related Art
Using balancing for charging/discharging batteries is known from prior art.
U.S. Pat. No. 6,114,835 for example discloses a balancing circuit that incorporates a voltage threshold which determines when to initiate a charge balance mode in order to equalize the level of charge in at least two cells of a multi-cell battery pack. Thereby balancing is initiated when the voltage level of a first cell reaches a first threshold voltage and balancing is stopped, when reaching a second threshold voltage. Balancing is used during charging a battery to balance the charge between cells. This can be done using passive balance circuits or active balance circuits. Normally the balancing method/balancing procedure is started at the end of charging, when one or more of the cells reach prematurely the end of charge voltage or during charging, when one cell voltage reaches a predefined fixed threshold. Then the balance circuit will equalize the charge of this cell compared to at least one of the other cells.
It is a drawback of this device that balancing is only applied between two specific thresholds and that it is only applicable for charging a battery. Furthermore the equalization takes a long time since the threshold is fixed at a certain value.
Patent application US2005/0140335 discloses a terminal voltage equalization circuit to equalize the terminal voltage of a series of connected battery strings. This patent application states that the charging current of the higher charge cells is reduced and the lower charged cells are equalized, making it therefore clear, that they use their concept during charge only. It is a further drawback that balancing is used on certain time intervals only.
According to prior art there are several methods and devices known to determine the state of health or the age of a battery. The internal battery resistance for example is a very good indicator for the state of health (or age) of the battery. Common methods for the internal resistance measurement are either the AC Impedance measurement, where a medium to high frequency signal is used to measure the impedance or the DC Resistance measurement, where with an additional circuit a DC current is used to measure the voltage drop and calculate the DC resistance. The latter is used off-line, meaning, that when the battery is not used, the cell is discharged with a constant current and the voltage drop is measured. To get an accurate reading the discharge current has to be in the order of half or full nominal battery current. This is not an energy problem as the pulses are quite short, but the measurement current has to be switched on (needs large semiconductor switch).